1. Field of the Invention
This invention relates to bimetallic alkoxide compounds useful as source reagents or precursors in chemical vapor deposition (CVD) processes such as those employed in the fabrication of ferroelectric and other oxide thin films.
2. Description of the Related Art
Ceramic materials have a wide range of useful properties, including optical transparency, semiconductivity, and superconductivity. Ferroelectric ceramic materials are especially interesting because of their high piezoelectric coefficients, high opto-electronic coefficients, high dielectric constants and high pyroelectric constants. Lead zirconate titanate (PZT) is an attractive ferroelectric capacitor material because of its large spontaneous polarization, low coercive field and high mechanical strength. It has also been well studied because of its extensive use in piezoelectric transducers.
Because of their great potential in microelectronics applications, thin film growth techniques for ferroelectrics such as PZT are required. The primary growth techniques used to deposit high quality ferroelectric thin films have been rf sputtering (Kwak, B. S.; Zhang, K.; Boyd, E. P.; Erbil, A.; Wilkens, B. J. J. Appl. Phys. 1991, 69 767) and metalorganic deposition (MOD). The latter involves thermal decomposition of soluble precursors from a solution phase. The high temperature post annealing procedures required here are unacceptable in actual device manufacture. With sputtering, as-deposited films have high coercive fields and low remnant polarizations and again require high temperature annealing to achieve good FE properties. High temperature annealing steps cannot be used in device processing schemes where the already formed structures would be destroyed by diffusion, for example, in a highly desirable application of ferroelectric thin films, as the dielectric elements in dynamic random access memories.
An alternative technique which has been very successful for semiconductor materials deposition is metalorganic chemical vapor deposition (MOCVD). The primary attraction of this technique is the potential for in-situ growth of the ferroelectric perovskite phase. Although more complicated than the MOD process, MOCVD is viewed as the technique that will ultimately be used for PZT device manufacture if the perovskite phase can be formed at temperatures compatible with other device materials (T&lt;450.degree. C.) and without the post anneal step, which is typically greater than 600.degree. C. Preliminary reports on the use of plasma enhanced-MOCVD have indeed described in-situ growth of the perovskite phase below 500.degree. C. Other advantages include the relatively simple scale-up to commercial production volumes as well as the superior step coverage of CVD.
The use of MOCVD for ferroelectric oxides has been limited by the lack of available metalorganic precursors and the toxicity associated with some of them (Brierley, C. J.; Trundle, C.; Considine, L.; Whatmore, R. W.; Ainger, F. W. Ferroelectrics 1989, 91, 181). While metal oxides have been successfully deposited using metal alkoxide precursors as an alternative to alkyls, there are only a few known for lead, and their volatilities are low.
The highest priority issue for MOCVD of complex oxides is control of stoichiometry. The conventional approach for compositional control over a quaternary oxide is to introduce metal-organic precursors to the reactor via three independently controlled manifolds, each requiring accurate control of temperature, pressure, flow rates and precursor concentrations. Besides its complexity, this method makes film stoichiometry highly sensitive to inaccuracies in any of these process variables.
In addition, problems are encountered in the preparation of the oxide ferroelectric materials in thin film form on diamond or other carbon-containing substrates. The methods for preparing these ferroelectric materials are well-known, including chemical vapor deposition, sputtering, and sol-gel processing. Sputtering is a preferred method (see e.g., "Epitaxial Growth and Electrical Properties of Ferroelectric Pb(Zr.sub.0.9 Ti.sub.0.1)O.sub.3 Films by Reactive Sputtering," Japan. J. Appl. Phys. B30, 1034-1037 (1991)). The ferroelectric oxide films are typically sputtered in the presence of oxygen, and thus the diamond substrate is liable to be oxidized. Therefore, a method for applying the films under milder conditions is needed.
While volatile materials for ferroelectrics such as lead titanate and lead zirconate titanate (and related doped analogs) are available, no sources exist which incorporate lead and titanium (or zirconium) in the desired 1:1 ratio directly. Also for the bulk manufacture of these oxides by "sol-gel" techniques and other thin film applications involving spin-on methods, a molecular precursor having a fixed lead-to-titanium(zirconium) ratio is desirable.
In general, lead titanate, lead zirconate and PZT have been made by mixture of separate precursors containing each of the required elements (c.f. Fukui et. al. J. Mater. Res. 1992, 7, 791). No bimetallic molecular species are known which contain lead and zirconium or lead and titanium in a 1:1 ratio. Only a few compounds containing lead and another metal have been previously reported. Mehrotra et. al. (Synth. React. Inorg. Met.-Org. Chem. 1989, 19, 195) reported the compound Pb[Sb(OR).sub.4 ].sub.2. Two reports of compounds that have 1:1 ratios of lead and titanium have appeared. Ramamurthi and Payne (Mat. Res. Soc. Proc. 1990, 180, 79) have proposed a compound that is polymeric, containing lead and titanium in an alkoxide framework. Yamaguchi et. al. (J. Am. Ceram. Soc. 1989, 72, 1660, and references therein) have synthesized PbTiO.sub.2 (OR).sub.2 by reaction of lead acetate and tetrabutoxytitanium. However these substances are not molecular species that contain only one atom of each metal, but rather have more than one empirical formula unit in each molecular formula. In each case, the relatively involatile product is not well defined as to molecular weight and vapor transport properties and is therefore not suitable as a CVD source reagent.
Accordingly, it is an object of the present invention to provide heterobimetallic source reagents containing two metals in the appropriate stoichiometric ratio required for the preparation of ceramic oxide films. Such reagents are sufficiently volatile to be used in CVD processes and decompose to deposit the desired oxide at temperatures that are low enough to be compatible with semiconductor device materials and processes.
It is another object of the present invention to provide methods of making these heterobimetallic source reagents.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.